When a flying spot CRT is operated for prolonged periods (several hours) the efficiency of the phosphor generally falls over those parts of the tube which have been scanned.
It is known to correct for the variations in brightness so produced in a film scanner by means of a photosensor directed at the face of the CRT, providing a correction signal, which is then divided into the video signal from the main photosensors (which receive light through the film being scanned). It is also known to avoid the horizontal bands which would otherwise result when the telecine is operating with the film moving (the scan height is reduced to compensate for the film movement), by slowly moving the scan up and down the CRT face and compensating for this motion by modulating the movement of the film. The scan may be moved by up to one half of the film frame height.
A particular aspect of this problem is that when the scan line is well focussed it can produce not just a patch on the phosphor, but will cause individual lines to be burned into the phosphor. Because of the close spacing these lines are not usually a problem in themselves, however if a different size of scan patch is subsequently applied with the same number of scanning lines, then a beat pattern results appearing as dark and light bands on the resultant picture and sometimes referred to as venetian blind effect. This effect could be corrected by the photosensor compensating system described above though this will itself introduce additional noise and shading and will not be fully effective for all colours. The previously described slow movement of the scan when the film is moving will avoid causing the burned in scan lines. However, it is common practice that a telecine be operated for long periods of time with the film stationary, when adjusting and assessing colour corrections for example.